Recently, air bag systems have become practically employed as a safety device for protecting the occupants of an automobile. Usually, an air bag which was produced by coating a surface of a smooth woven fabric with a chloroprene rubber or silicone rubber, cutting the coated fabric into a circular form, superposing two cut fabric pieces on each other and seam-joining the fabric pieces at the circumferential edge portions thereof, is used.
Since the air bag is used by inflating it with a gas blown in an instant thereinto, the air bag is required to have no risk of bursting by a rapid raising of the inside pressure thereof. For meeting the requirement, various attempts were made. For example, Japanese Unexamined Patent Publication (Kokai) No. 2-90,170 attempted to provide an air bag produced by preparing non-circular woven fabric pieces having larger distances between a center point and a circumferential edge thereof measured in longitudinal directions of the warp and weft yarns than distances between the center and the circumferential edge thereof measured in a direction inclined at an angle of from 40 to 50 degrees from the longitudinal direction of the warp yarns to the longitudinal direction of the weft yarns; superposing the non-circular woven fabric pieces on each other; and seam-joining the pieces at the circumferential edge portions thereof.
Also, Japanese Unexamined Patent Publication (Kokai) No. 4-5,145 discloses a non-circular air bag made from hollow weave fabrics having an axis length thereof in a direction at a bias angle of 45 degrees from the warp direction, corresponding to 70 to 95% of the axis length thereof in the warp and weft direction.
Further, Japanese Unexamined Patent Publication (Kokai) No. 4-43,143 discloses an air bag made from non-circular hollow weave fabric pieces having a longer axis in a direction inclined at a bias angle from the warp or weft direction than an axis in the warp or weft direction of the fabric pieces.
The above-mentioned attempts are based on the idea that a starting point in bursting of the air bag resides in a seam-joining portion in a circumferential edge portion of the air bag, and provided an air bag made from non-circular woven fabric pieces having a shorter axis in a bias direction than that in the warp or weft direction, to enhance the burst strength of the seam-joining portion.
In another attempt, not using non-circular woven fabric pieces, two circular woven fabric pieces as mentioned above are superposed on each other and seam-jointed at circumferential edge portions thereof to each other. In this attempt, a portion of an inflator side woven fabric piece, namely of a bottom cloth, surrounding a hole for connecting an inflator to the air bag therethrough, is reinforced with a woven fabric apron, to prevent a bursting and perforating of the bottom cloth by the inflation gas.
Nevertheless, the above-mentioned prior art air bags are not satisfactory in burst strength, and thus a further enhancement of the burst strength is required. Especially, the demand is very strong for lightweight, compact air bags.